Enhanced subcortical spreading depression in familial hemiplegic migraine type 1 mutant mice

In vivo imaging reveals that pregabalin inhibits cortical spreading depression and propagation to subcortical brain structures

Migraine is characterized by severe headaches that can be preceded by an aura likely caused by cortical spreading depression (SD). The antiepileptic pregabalin (Lyrica) shows clinical promise for migraine therapy, although its efficacy and mechanism of action are unclear. As detected by diffusion-weighted MRI (DW-MRI) in wild-type (WT) mice, the acute systemic administration of pregabalin increased the threshold for SD initiation in vivo. In familial hemiplegic migraine type 1 mutant mice expressing human mutations (R192Q and S218L) in the Ca_V2.1 (P/Q-type) calcium channel subunit, pregabalin slowed the speed of SD propagation in vivo. Acute systemic administration of pregabalin in vivo also selectively prevented the migration of SD into subcortical striatal and hippocampal regions in the R192Q strain that exhibits a milder phenotype and gain of Ca_V2.1 channel function. At the cellular level, pregabalin inhibited glutamatergic synaptic transmission differentially in WT, R192Q, and S218L mice. The study describes a DW-MRI analysis method for tracking the progression of SD and provides support and a mechanism of action for pregabalin as a possible effective therapy in the treatment of migraine.

Vagus nerve stimulation inhibits cortical spreading depression

Vagus nerve stimulation has recently been reported to improve symptoms of migraine. Cortical spreading depression is the electrophysiological event underlying migraine aura and is a trigger for headache. We tested whether vagus nerve stimulation inhibits cortical spreading depression to explain its antimigraine effect. Unilateral vagus nerve stimulation was delivered either noninvasively through the skin or directly by electrodes placed around the nerve. Systemic physiology was monitored throughout the study. Both noninvasive transcutaneous and invasive direct vagus nerve stimulations significantly suppressed spreading depression susceptibility in the occipital cortex in rats. The electrical stimulation threshold to evoke a spreading depression was elevated by more than 2-fold, the frequency of spreading depressions during continuous topical 1 M KCl was reduced by ∼40%, and propagation speed of spreading depression was reduced by ∼15%. This effect developed within 30 minutes after vagus nerve stimulation and persisted for more than 3 hours. Noninvasive transcutaneous vagus nerve stimulation was as efficacious as direct invasive vagus nerve stimulation, and the efficacy did not differ between the ipsilateral and contralateral hemispheres. Our findings provide a potential mechanism by which vagus nerve stimulation may be efficacious in migraine and suggest that susceptibility to spreading depression is a suitable platform to optimize its efficacy.

Leaky RyR2 channels unleash a brainstem spreading depolarization mechanism of sudden cardiac death

Cardiorespiratory failure is the most common cause of sudden unexplained death in epilepsy (SUDEP). Genetic autopsies have detected "leaky" gain-of-function mutations in the ryanodine receptor-2 (RyR2) gene in both SUDEP and sudden cardiac death cases linked to catecholaminergic polymorphic ventricular tachycardia that feature lethal cardiac arrhythmias without structural abnormality. Here we find that a human leaky RyR2 mutation, R176Q (RQ), alters neurotransmitter release probability in mice and significantly lowers the threshold for spreading depolarization (SD) in dorsal medulla, leading to cardiorespiratory collapse. Rare episodes of sinus bradycardia, spontaneous seizure, and sudden death were detected in RQ/+ mutant mice in vivo; however, when provoked, cortical seizures frequently led to apneas, brainstem SD, cardiorespiratory failure, and death. In vitro studies revealed that the RQ mutation selectively strengthened excitatory, but not inhibitory, synapses and facilitated SD in both the neocortex as well as brainstem dorsal medulla autonomic microcircuits. These data link defects in neuronal intracellular calcium homeostasis to the vulnerability of central autonomic brainstem pathways to hypoxic stress and implicate brainstem SD as a previously unrecognized site and mechanism contributing to premature death in individuals with leaky RYR2 mutations.

Spreading Depolarizations: A Therapeutic Target Against Delayed Cerebral Ischemia After Subarachnoid Hemorrhage

Delayed cerebral ischemia is the most feared cause of secondary injury progression after subarachnoid hemorrhage. Initially thought to be a direct consequence of large artery spasm and territorial ischemia, recent data suggests that delayed cerebral ischemia represents multiple concurrent and synergistic mechanisms, including microcirculatory dysfunction, inflammation, and microthrombosis. Among these mechanisms, spreading depolarizations (SDs) are arguably the most elusive and underappreciated in the clinical setting. Although SDs have been experimentally detected and examined since the late 1970s, their widespread occurrence in human brain was not unequivocally demonstrated until relatively recently. We now know that SDs occur with very high incidence in human brain after ischemic or hemorrhagic stroke and trauma, and worsen outcomes by increasing metabolic demand, decreasing blood supply, predisposing to seizure activity, and possibly worsening brain edema. In this review, we discuss the causes and consequences of SDs in injured brain. Although much of our mechanistic knowledge comes from experimental models of focal cerebral ischemia, clinical data suggest that the same principles apply regardless of the mode of injury (i.e., ischemia, hemorrhage, or trauma). The hope is that a better fundamental understanding of SDs will lead to novel therapeutic interventions to prevent SD occurrence and its adverse consequences contributing to injury progression in subarachnoid hemorrhage and other forms of acute brain injury.

Recurrent coma and fever in familial hemiplegic migraine type 2. A prospective 15-year follow-up of a large family with a novel ATP1A2 mutation

Background Familial hemiplegic migraine (FHM) is a rare monogenic migraine subtype characterised by attacks associated with transient motor weakness. Clinical information is mainly based on reports of small families with only short follow-up. Here, we document a prospective 15-year follow-up of an extended family with FHM type 2. Patients and methods After diagnosing FHM in a patient with severe attacks associated with coma and fever, we identified eight more family members with FHM and one with possible FHM. All family members were prospectively followed for 15 years. In total 13 clinically affected and 21 clinically non-affected family members were genetically tested and repeatedly investigated. Results A novel p.Arg348Pro ATP1A2 mutation was found in 14 family members: 12 with clinical FHM, one with psychomotor retardation and possible FHM, and one without FHM features. In 9/12 (75%) family members with genetically confirmed FHM, attacks were severe, long-lasting, and often associated with impaired consciousness and fever. Such attacks were frequently misdiagnosed and treated as viral meningitis or stroke. Epilepsy was reported in three family members with FHM and in the one with psychomotor retardation and possible FHM. Ataxia was not observed. Conclusion FHM should be considered in patients with recurrent coma and fever.

Animal models of monogenic migraine

Migraine is a highly prevalent and disabling neurological disorder with a strong genetic component. Rare monogenic forms of migraine, or syndromes in which migraine frequently occurs, help scientists to unravel pathogenetic mechanisms of migraine and its comorbidities. Transgenic mouse models for rare monogenic mutations causing familial hemiplegic migraine (FHM), cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and familial advanced sleep-phase syndrome (FASPS), have been created. Here, we review the current state of research using these mutant mice. We also discuss how currently available experimental approaches, including epigenetic studies, biomolecular analysis and optogenetic technologies, can be used for characterization of migraine genes to further unravel the functional and molecular pathways involved in migraine.

Susceptibility of Primary Sensory Cortex to Spreading Depolarizations

Spreading depolarizations (SDs) are recognized as actors in neurological disorders as diverse as migraine and traumatic brain injury (TBI). Migraine aura involves sensory percepts, suggesting that sensory cortices might be intrinsically susceptible to SDs. We used optical imaging, MRI, and field potential and potassium electrode recordings in mice and electrocorticographic recordings in humans to determine the susceptibility of different brain regions to SDs. Optical imaging experiments in mice under isoflurane anesthesia showed that both cortical spreading depression and terminal anoxic depolarization arose preferentially in the whisker barrel region of parietal sensory cortex. MRI recordings under isoflurane, ketamine/xylazine, ketamine/isoflurane, and urethane anesthesia demonstrated that the depolarizations did not propagate from a subcortical source. Potassium concentrations showed larger increases in sensory cortex, suggesting a mechanism of susceptibility. Sensory stimulation biased the timing but not the location of depolarization onset. In humans with TBI, there was a trend toward increased incidence of SDs in parietal/temporal sensory cortex compared with other regions. In conclusion, SDs are inducible preferentially in primary sensory cortex in mice and most likely in humans. This tropism can explain the predominant sensory phenomenology of migraine aura. It also demonstrates that sensory cortices are vulnerable in brain injury.

SIGNIFICANCE STATEMENT

Spreading depolarizations (SDs) are involved in neurologic disorders as diverse as migraine and traumatic brain injury. In migraine, the nature of aura symptoms suggests that sensory cortex may be preferentially susceptible. In brain injury, SDs occur at a vulnerable time, during which the issue of sensory stimulation is much debated. We show, in mouse and human, that sensory cortex is more susceptible to SDs. We find that sensory stimulation biases the timing but not the location of the depolarizations. Finally, we show a relative impairment of potassium clearance in sensory cortex, providing a potential mechanism for the susceptibility. Our data help to explain the sensory nature of the migraine aura and reveal that sensory cortices are vulnerable in brain injury.

Triptans disrupt brain networks and promote stress-induced CSD-like responses in cortical and subcortical areas

A number of drugs, including triptans, promote migraine chronification in susceptible individuals. In rats, a period of triptan administration over 7 days can produce "latent sensitization" (14 days after discontinuation of drug) demonstrated as enhanced sensitivity to presumed migraine triggers such as environmental stress and lowered threshold for electrically induced cortical spreading depression (CSD). Here we have used fMRI to evaluate the early changes in brain networks at day 7 of sumatriptan administration that may induce latent sensitization as well as the potential response to stress. After continuous infusion of sumatriptan, rats were scanned to measure changes in resting state networks and the response to bright light environmental stress. Rats receiving sumatriptan, but not saline infusion, showed significant differences in default mode, autonomic, basal ganglia, salience, and sensorimotor networks. Bright light stress produced CSD-like responses in sumatriptan-treated but not control rats. Our data show the first brain-related changes in a rat model of medication overuse headache and suggest that this approach could be used to evaluate the multiple brain networks involved that may promote this condition.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

GluN2A/B ratio elevation induced by cortical spreading depression: electrophysiological and quantitative studies of the hippocampus

Cortical spreading depression (CSD), an underlying mechanism of migraine aura, propagates to the hippocampus, and might explain hippocampusassociated symptoms during migraine attack. We hypothesised that this process is, some parts, mediated by NMDA receptors. By using a rat model, CSD was elicited by solid KCl for 45 minutes prior to electrophysiological and quantitative analyses. The result from electrophysiological study was the ratio of glutamate NMDA receptor 2A and 2B subunits (GluN2A/B). Total NMDA receptor response was isolated using an AMPA antagonist, prior to a GluN2B receptor antagonist. The GluN2A/B ratio was calculated by dividing the remaining NMDA-mediated field-excitatory synaptic potentials (fEPSP) with the subtracted difference of NMDAmediated fEPSP. Western blot analysis of the hippocampus was performed to confirm the quantitative change of GluN2A/B ratio. In hippocampal slice study (n = 12), the GluN2A/B ratio of hippocampal fEPSP was significantly increased in CSD group. Western blot analysis (n = 30) revealed an increase in GluN2A subunits and a decrease in GluN2B subunits in the hippocampus ipsilateral to the CSD induction. Our current study revealed that GluN2A/B ratio was shown to be elevated following CSD stimulation by increasing the total number of GluN2A while reducing the total number of GluN2B subunits. This ratio was demonstrated to be associated with synaptic plasticity of the hippocampus in numerous studies. In conclusion, we showed that CSD increased GluN2A/B ratio, in turn, would result in altered synaptic plasticity. Our findings provide a probable implication on the correlation of migraine aura and hippocampusassociated symptoms.

Migraine mutations impair hippocampal learning despite enhanced long-term potentiation

To explain cognitive and memory difficulties observed in some familial hemiplegic migraine (FHM) patients, we examined hippocampal neurotransmission and plasticity in knock-in mice expressing the FHM type 1 (FHM1) R192Q gain-of function mutation in the CACNA1A gene that encodes the α1A subunit of neuronal CaV2.1 channels. We determined stimulus intensity-response curves for anterior commissure-evoked hippocampal CA1 field potentials in strata pyramidale and radiatum and assessed neuroplasticity by inducing long-term potentiation (LTP) and long-term depression (LTD) in anesthetized mice in vivo. We also studied learning and memory using contextual fear-conditioning, Morris water maze, and novel object recognition tests. Hippocampal field potentials were significantly enhanced in R192Q mice compared with wild-type controls. Stimulus intensity-response curves were shifted to the left and displayed larger maxima in the mutants. LTP was augmented by twofold in R192Q mice, whereas LTD was unchanged compared with wild-type mice. R192Q mice showed significant spatial memory deficits in contextual fear-conditioning and Morris water maze tests compared with wild-type controls. Novel object recognition was not impaired in R192Q mice; however, mice carrying the more severe S218L CACNA1A mutation showed marked deficits in this test, suggesting a genotype-phenotype relationship. Thus, whereas FHM1 gain-of-function mutations enhance hippocampal excitatory transmission and LTP, learning and memory are paradoxically impaired, providing a possible explanation for cognitive changes detected in FHM. Data suggest that abnormally enhanced plasticity can be as detrimental to efficient learning as reduced plasticity and highlight how genetically enhanced neuronal excitability may impact cognitive function.

Large-scale mass spectrometry imaging investigation of consequences of cortical spreading depression in a transgenic mouse model of migraine

Cortical spreading depression (CSD) is the electrophysiological correlate of migraine aura. Transgenic mice carrying the R192Q missense mutation in the Cacna1a gene, which in patients causes familial hemiplegic migraine type 1 (FHM1), exhibit increased propensity to CSD. Herein, mass spectrometry imaging (MSI) was applied for the first time to an animal cohort of transgenic and wild type mice to study the biomolecular changes following CSD in the brain. Ninety-six coronal brain sections from 32 mice were analyzed by MALDI-MSI. All MSI datasets were registered to the Allen Brain Atlas reference atlas of the mouse brain so that the molecular signatures of distinct brain regions could be compared. A number of metabolites and peptides showed substantial changes in the brain associated with CSD. Among those, different mass spectral features showed significant (t-test, P < 0.05) changes in the cortex, 146 and 377 Da, and in the thalamus, 1820 and 1834 Da, of the CSD-affected hemisphere of FHM1 R192Q mice. Our findings reveal CSD- and genotype-specific molecular changes in the brain of FHM1 transgenic mice that may further our understanding about the role of CSD in migraine pathophysiology. The results also demonstrate the utility of aligning MSI datasets to a common reference atlas for large-scale MSI investigations.

Identification of the extent of cortical spreading depression propagation by Npas4 mRNA expression

Cortical spreading depression (CSD) is a phenomenon associated with a propagating large shift in direct current (DC) potential followed by suppression of electrophysiological activity. For temporal analysis of CSD propagation, electrophysiological recording is the most reliable tool. However, it is difficult to completely identify the spatial area of the brain influenced by CSD, because recording sites are technically limited. Histological post hoc identification of activated neurons by labeling the induction of an immediate early gene (IEG) could determine areas of CSD propagation. We found that cortical application of potassium chloride induced expression of Npas4 IEG mRNA in the ipsilateral dorsal cortex. Interestingly, induction of Npas4 was never observed in the ipsilateral hippocampus and there was a clear boundary to the area of Npas4 expression. To determine whether the boundary of the area of Npas4 mRNA expression was the limit of CSD propagation, we recorded local field potentials from multiple sites that crossed the boundary of Npas4 expression. We found that the area of Npas4 mRNA expression coincided with the area of DC-potential shift propagation. We propose that induction of Npas4 identifies the area influenced by CSD propagation.

Migraine pathophysiology: lessons from mouse models and human genetics

Migraine is a common, disabling, and undertreated episodic brain disorder that is more common in women than in men. Unbiased genome-wide association studies have identified 13 migraine-associated variants pointing at genes that cluster in pathways for glutamatergic neurotransmission, synaptic function, pain sensing, metalloproteinases, and the vasculature. The individual pathogenetic contribution of each gene variant is difficult to assess because of small effect sizes and complex interactions. Six genes with large effect sizes were identified in patients with rare monogenic migraine syndromes, in which hemiplegic migraine and non-hemiplegic migraine with or without aura are part of a wider clinical spectrum. Transgenic mouse models with human monogenic-migraine-syndrome gene mutations showed migraine-like features, increased glutamatergic neurotransmission, cerebral hyperexcitability, and enhanced susceptibility to cortical spreading depression, which is the electrophysiological correlate of aura and a putative trigger for migraine. Enhanced susceptibility to cortical spreading depression increased sensitivity to focal cerebral ischaemia, and blocking of cortical spreading depression improved stroke outcome in these mice. Changes in female hormone levels in these mice modulated cortical spreading depression susceptibility in much the same way that hormonal fluctuations affect migraine activity in patients. These findings confirm the multifactorial basis of migraine and might allow new prophylactic options to be developed, not only for migraine but potentially also for migraine-comorbid disorders such as epilepsy, depression, and stroke.

Biphasic neurovascular changes in prolonged migraine aura in familial hemiplegic migraine type 2

OBJECTIVE

To report biphasic changes in cerebral blood flow (CBF) in the acute phase of hemiplegic migraine with prolonged aura (HMPA), in which aura symptoms lasted longer than 24 h, in three patients with familial hemiplegic migraine (FHM) carrying a p.H916L mutation in ATP1A2 gene.

METHODS

We assessed neurovascular changes with time in the affected cerebral hemisphere corresponding to aura symptoms during the acute phase of HMPA. Arterial spin labelling MRI, SPECT for CBF measurement and EEG in three attacks, in one attack FDG-PET measurement for cerebral metabolism was performed. We evaluated CBF at different phases of aura symptoms in 11 attacks of HMPA.

RESULTS

In two attacks, we found biphasic CBF changes beginning with hypoperfusion followed by persistent hyperperfusion. FDG-PET revealed increased cerebral glucose metabolism in the regions corresponding to hyperperfusion on day 4 when aura symptoms still persisted. In four attacks, Z-score-based CBF mapping revealed multifocal hypoperfusion in the early phase. Hypoperfusion in our study was seen within 19 h of the onset of the symptoms in five of seven attacks, while hyperperfusion was seen 18 h or later in eight of nine attacks. EEG showed attenuated alpha activity without paroxysmal discharge.

CONCLUSIONS

This is the first report showing biphasic CBF changes during the prolonged aura of FHM2. This study suggested that the results of cross-sectional CBF studies should be interpreted carefully. Initial multifocal hypoperfusion is likely due to functional depression of multifocal origin in the affected hemisphere, but the mechanism of persistent hyperperfusion requires further investigation.

Abnormal cortical synaptic transmission in CaV2.1 knockin mice with the S218L missense mutation which causes a severe familial hemiplegic migraine syndrome in humans

Familial hemiplegic migraine type 1 (FHM1) is caused by gain-of-function mutations in Ca_V2.1 (P/Q-type) Ca²⁺ channels. Knockin (KI) mice carrying the FHM1 R192Q missense mutation show enhanced cortical excitatory synaptic transmission at pyramidal cell synapses but unaltered cortical inhibitory neurotransmission at fast-spiking interneuron synapses. Enhanced cortical glutamate release was shown to cause the facilitation of cortical spreading depression (CSD) in R192Q KI mice. It, however, remains unknown how other FHM1 mutations affect cortical synaptic transmission. Here, we studied neurotransmission in cortical neurons in microculture from KI mice carrying the S218L mutation, which causes a severe FHM syndrome in humans and an allele-dosage dependent facilitation of experimental CSD in KI mice, which is larger than that caused by the R192Q mutation. We show gain-of-function of excitatory neurotransmission, due to increased action-potential evoked Ca²⁺ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at multipolar interneuron synapses in S218L KI mice. In contrast with the larger gain-of-function of neuronal Ca_V2.1 current in homozygous than heterozygous S218L KI mice, the gain-of-function of evoked glutamate release, the paired-pulse ratio and the Ca²⁺ dependence of the excitatory postsynaptic current were similar in homozygous and heterozygous S218L KI mice, suggesting compensatory changes in the homozygous mice. Furthermore, we reveal a unique feature of S218L KI cortical synapses which is the presence of a fraction of mutant Ca_V2.1 channels being open at resting potential. Our data suggest that, while the gain-of-function of evoked glutamate release may explain the facilitation of CSD in heterozygous S218L KI mice, the further facilitation of CSD in homozygous S218L KI mice is due to other Ca_V2.1-dependent mechanisms, that likely include Ca²⁺ influx at voltages sub-threshold for action potential generation.

Spreading depression transiently disrupts myelin via interferon-gamma signaling

Multiple sclerosis and migraine with aura are clinically correlated and both show imaging changes suggestive of myelin disruption. Furthermore, cortical myelin loss in the cuprizone animal model of multiple sclerosis enhances susceptibility to spreading depression, the likely underlying cause of migraine with aura. Since multiple sclerosis pathology involves inflammatory T cell lymphocyte production of interferon-gamma and a resulting increase in oxidative stress, we tested the hypothesis that spreading depression disrupts myelin through similar signaling pathways. Rat hippocampal slice cultures were initially used to explore myelin loss in spreading depression, since they contain T cells, and allow for controlled tissue microenvironment. These experiments were then translated to the in vivo condition in neocortex. Spreading depression in slice cultures induced significant loss of myelin integrity and myelin basic protein one day later, with gradual recovery by seven days. Myelin basic protein loss was abrogated by T cell depletion, neutralization of interferon-gamma, and pharmacological inhibition of neutral sphingomyelinase-2. Conversely, one day after exposure to interferon-gamma, significant reductions in spreading depression threshold, increases in oxidative stress, and reduced levels of glutathione, an endogenous neutral sphingomyelinase-2 inhibitor, emerged. Similarly, spreading depression triggered significant T cell accumulation, sphingomyelinase activation, increased oxidative stress, and reduction of gray and white matter myelin in vivo. Myelin disruption is involved in spreading depression, thereby providing pathophysiological links between multiple sclerosis and migraine with aura. Myelin disruption may promote spreading depression by enhancing aberrant excitability. Thus, preservation of myelin integrity may provide novel therapeutic targets for migraine with aura.

Migraine genetics: current findings and future lines of research

In the last two decades, migraine research has greatly advanced our current knowledge of the genetic contributions and the pathophysiology of this common and debilitating disorder. Nonetheless, this knowledge still needs to grow further and to translate into more effective treatments. To date, several genes involved in syndromic and monogenic forms of migraine have been identified, allowing the generation of animal models which have significantly contributed to current knowledge of the mechanisms underlying these rare forms of migraine. Common forms of migraine are instead posing a greater challenge, as they may most often stem from complex interactions between multiple common genetic variants, with environmental triggers. This paper reviews our current understanding of migraine genetics, moving from syndromic and monogenic forms to oligogenic/polygenic migraines most recently addressed with some success through genome-wide association studies. Methodological issues in study design and future perspectives opened by biomarker research will also be briefly addressed.

Migraine and stroke: in search of shared mechanisms

BACKGROUND

Migraine, particularly with aura, increases the risk for ischemic stroke, at least in a subset of patients. The underlying mechanisms are poorly understood and probably multifactorial.

METHODS

We carried out an extended literature review of experimental and clinical evidence supporting the association between migraine and ischemic stroke to identify potential mechanisms that can explain the association.

RESULTS

Observational, imaging and genetic evidence support a link between migraine and ischemic stroke. Based on clinical and experimental data, we propose mechanistic hypotheses to explain the link, such as microembolic triggers of migraine and enhanced sensitivity to ischemic injury in migraineurs.

DISCUSSION

We discuss the possible practical implications of clinical and experimental data, such as aggressive risk factor screening and management, stroke prophylaxis and specific acute stroke management in migraineurs. However, evidence from prospective clinical trials is required before modifying the practice in this patient population.

Propagation of cortical spreading depression into the hippocampus: The role of the entorhinal cortex

Propagation of cortical spreading depression (CSD) to the subcortical structures could be the underlying mechanism of some neurological deficits in migraine with aura. The entorhinal cortex (EC) as a gray matter bridge between the neocortex and subcortical regions plays an important role in this propagation. In vitro combined neocortex-hippocampus brain slices were used to study the propagation pattern of CSD between the neocortex and the hippocampus. The effects of different compounds as well as tetanic electrical stimulations in the EC on propagation of CSD to the hippocampus were investigated. Repetitive induction of CSD by KCl injection in the somatosensory cortex enhanced the probability of CSD entrance to the hippocampus via EC. Local application of AMPA receptor blocker CNQX and cannabinoid receptor agonist WIN 55212-2 in EC facilitated the propagation of CSD to the hippocampus, whereas application of NMDA receptor blocker APV and GABA_A receptor blocker bicuculline in this region reduced the probability of CSD penetration to the hippocampus. Application of tetanic stimulation in EC also facilitated the propagation of CSD entrance to the hippocampus. Our data suggest the importance of synaptic plasticity of EC in filtering the propagation of CSD into subcortical structures and possibly the occurrence of concomitant neurological deficits. Synapse 68:574-584, 2014. © 2014 Wiley Periodicals, Inc.

Synaptic gain-of-function effects of mutant Cav2.1 channels in a mouse model of familial hemiplegic migraine are due to increased basal [Ca2+]i

Specific missense mutations in the CACNA1A gene, which encodes a subunit of voltage-gated CaV2.1 channels, are associated with familial hemiplegic migraine type 1 (FHM1), a rare monogenic subtype of common migraine with aura. We used transgenic knock-in (KI) mice harboring the human pathogenic FHM1 mutation S218L to study presynaptic Ca(2+) currents, EPSCs, and in vivo activity at the calyx of Held synapse. Whole-cell patch-clamp recordings of presynaptic terminals from S218L KI mice showed a strong shift of the calcium current I-V curve to more negative potentials, leading to an increase in basal [Ca(2+)]i, increased levels of spontaneous transmitter release, faster recovery from synaptic depression, and enhanced synaptic strength despite smaller action-potential-elicited Ca(2+) currents. The gain-of-function of transmitter release of the S218L mutant was reproduced in vivo, including evidence for an increased release probability, demonstrating its relevance for glutamatergic transmission. This synaptic phenotype may explain the misbalance between excitation and inhibition in neuronal circuits resulting in a persistent hyperexcitability state and other migraine-relevant mechanisms such as an increased susceptibility to cortical spreading depression.

Chaos and commotion in the wake of cortical spreading depression and spreading depolarizations

Punctuated episodes of spreading depolarizations erupt in the brain, encumbering tissue structure and function, and raising fascinating unanswered questions concerning their initiation and propagation. Linked to migraine aura and headache, cortical spreading depression contributes to the morbidity in the world's migraine with aura population. Even more ominously, erupting spreading depolarizations accelerate tissue damage during brain injury. The once-held view that spreading depolarizations may not exist in the human brain has changed, largely because of the discovery of migraine genes that confer cortical spreading depression susceptibility, the application of sophisticated imaging tools and efforts to interrogate their impact in the acutely injured human brain.

Spreading depolarization may link migraine and stroke

Migraine increases the risk of stroke, particularly in young and otherwise healthy adults. Being the most frequent neurological condition, migraine prevalence is on a par with that of other common stroke risk factors, such as diabetes or hypertension. Several patterns of association have emerged: (1) migraine and stroke share a common association (eg, vasculopathies, patent foramen ovale, or pulmonary A-V malformations); (2) injury to the arterial wall such as acute arterial dissections can present as migraine aura attacks or stroke; (3) strokes rarely develop during a migraine attack, as described for "migrainous stroke." Increasing experimental evidence suggests that cerebral hyperexcitability and enhanced susceptibility to spreading depolarization, the electrophysiologic event underlying migraine, may serve as a mechanism underlying the migraine-stroke association. Mice carrying human vascular or neuronal migraine mutations exhibit an enhanced susceptibility to spreading depolarization while being particularly vulnerable to cerebral ischemia. The severe stroke phenotype in migraine mutant mice can be prevented by suppressing spreading depolarization. If confirmed in the clinical setting, inhibiting spreading depolarization might protect migraineurs at stroke risk as well as decrease attacks of migraine.

Ophthalmoplegia starting with a headache circumscribed in a line-shaped area: a subtype of ophthalmoplegic migraine?

Recurrent painful ophthalmoplegic neuropathy (RPON), formerly named ophthalmoplegic migraine (OM), is a rare condition characterized by the association of unilateral headaches and the ipsilateral oculomotor nerve palsy. The third cranial nerve is most commonly involved in the recurrent attacks. But it is still debated whether a migraine or an oculomotor neuropathy may be the primary cause of this disorder. Here, we report an elder patient who had a recurrent ophthalmoplegia starting with an unilateral headache circumscribed in an area shaped in a line linking the posterior-parietal region and the ipsilateral eye. And the headache had couple of features similar to that of migraine, such as past history of recurrent migraine attacks, accompaniments of nausea, vomiting, and phonophobia, response to flunarizine and sodium valproate. We may herein report a subtype of OM but not a RPON. This case report indicates that OM may exist as an entity and some OM may be wrongly grouped under the category of RPON in the current international headache classification.

Structural abnormalities in the thalamus of migraineurs with aura: a multiparametric study at 3 T

BACKGROUND AND OBJECTIVES

The thalamus exerts a pivotal role in pain processing and cortical excitability control, and migraine is characterized by repeated pain attacks and abnormal cortical habituation to excitatory stimuli. This work aimed at studying the microstructure of the thalamus in migraine patients using an innovative multiparametric approach at high-field magnetic resonance imaging (MRI).

DESIGN

We examined 37 migraineurs (22 without aura, MWoA, and 15 with aura, MWA) as well as 20 healthy controls (HC) in a 3-T MRI equipped with a 32-channel coil. We acquired whole-brain T1 relaxation maps and computed magnetization transfer ratio (MTR), generalized fractional anisotropy, and T2* maps to probe microstructural and connectivity integrity and to assess iron deposition. We also correlated the obtained parametric values with the average monthly frequency of migraine attacks and disease duration.

RESULTS

T1 relaxation time was significantly shorter in the thalamus of MWA patients compared with MWoA (P < 0.001) and HC (P ≤ 0.01); in addition, MTR was higher and T2* relaxation time was shorter in MWA than in MWoA patients (P < 0.05, respectively). These data reveal broad microstructural alterations in the thalamus of MWA patients compared with MWoA and HC, suggesting increased iron deposition and myelin content/cellularity. However, MWA and MWoA patients did not show any differences in the thalamic nucleus involved in pain processing in migraine.

CONCLUSIONS

There are broad microstructural alterations in the thalamus of MWA patients that may underlie abnormal cortical excitability control leading to cortical spreading depression and visual aura.

Calcium channels and synaptic transmission in familial hemiplegic migraine type 1 animal models

One of the outstanding developments in clinical neurology has been the identification of ion channel mutations as the origin of a wide variety of inherited disorders like migraine, epilepsy, and ataxia. The study of several channelopathies has provided crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological diseases. This review addresses the mutations underlying familial hemiplegic migraine (FHM) with particular interest in Cav2.1 (i.e., P/Q-type) voltage-activated Ca²⁺ channel FHM type-1 mutations (FHM1). Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of Cav2.1 channels. FHM1 Cav2.1 channels activate at more hyperpolarizing potentials and show an increased open probability. These biophysical alterations may lead to a gain-of-function on synaptic transmission depending upon factors such as action potential waveform and/or Cav2.1 splice variants and auxiliary subunits. Analysis of FHM knock-in mouse models has demonstrated a deficient regulation of the cortical excitation/inhibition (E/I) balance. The resulting excessive increases in cortical excitation may be the mechanisms that underlie abnormal sensory processing together with an increase in the susceptibility to cortical spreading depression (CSD). Increasing evidence from FHM KI animal studies support the idea that CSD, the underlying mechanism of aura, can activate trigeminal nociception, and thus trigger the headache mechanisms.

Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?

Adult neurogenesis, the birth of new neurons in the mature brain, has attracted considerable attention in the last decade. One of the earliest identified and most profound factors that affect adult neurogenesis both positively and negatively is stress. Here, we review the complex interplay between stress and adult neurogenesis. In particular, we review the role of the glucocorticoid receptor, the main mediator of the stress response in the proliferation, differentiation, migration, and functional integration of newborn neurons in the hippocampus. We review a multitude of mechanisms regulating glucocorticoid receptor activity in relationship to adult neurogenesis. We postulate a novel concept in which the level of glucocorticoid receptor expression directly regulates the excitation-inhibition balance, which is key for proper neurogenesis. We furthermore argue that an excitation-inhibition dis-balance may underlie aberrant functional integration of newborn neurons that is associated with psychiatric and paroxysmal brain disorders.

Pearls and pitfalls in experimental models of spreading depression

BACKGROUND

Spreading depression (SD) is the electrophysiological substrate of migraine aura and a potential trigger for headache. Since its discovery by Leão in 1944, SD has transformed from being viewed as an epiphenomenon into a therapeutic target relevant in the pathophysiology of migraine and brain injury.

AIM

Despite decades of research, the underpinnings of SD are still poorly understood, hampering our efforts to selectively block its initiation and spread. Experimental models have nevertheless been useful to measure the likelihood of SD occurrence (i.e. SD susceptibility) and characterize genetic, physiological and pharmacological modulation of SD in search of potential therapies, such as in migraine prophylaxis and stroke. Here, I review experimental SD susceptibility endpoints and surrogates, and minimum essential model requirements to improve their utility in drug screening.

CONCLUSION

A critical reappraisal of strengths and caveats of experimental models of SD susceptibility is needed to set standards and improve data quality, interpretation and reconciliation.

Familial hemiplegic migraine and spreading depression

Familial hemiplegic migraine (FHM) is an autosomal dominantly inherited subtype of migraine with aura, characterized by transient neurological signs and symptoms. Typical hemiplegic migraine attacks start in the first or second decade of life. Some patients with FHM suffer from daily recurrent attacks since childhood. Results from extensive studies of cellular and animal models have indicated that gene mutations in FHM increase neuronal excitability and reduce the threshold for spreading depression (SD). SD is a transient wave of profound neuronal and glial depolarization that slowly propagates throughout the brain tissue and is characterized by a high amplitude negative DC shift. After induction of SD, S218L mutant mice exhibited neurological signs highly reminiscent of clinical attacks in FHM type 1 patients carrying this mutation. FHM1 with ataxia is attributable to specific mutations that differ from mutations that cause pure FHM1 and have peculiar consequences on cerebellar Cav2.1 currents that lead to profound Purkinje cell dysfunction and neuronal loss with atrophy. SD in juvenile rats produced neuronal injury and death. Hormonal factors involved in FHM affect SD initiation and propagation. The data identify SD as a possible target of treatment of FHM. In addition, FHM is a useful model to explore the mechanisms of more common types of migraine.

Cortical spreading depression and migraine

Cortical spreading depression (CSD), a slowly propagated wave of depolarization followed by suppression of brain activity, is a remarkably complex event that involves dramatic changes in neural and vascular function. Since its original description in the 1940s, CSD has been hypothesized to be the underlying mechanism of the migraine aura. Substantial evidence from animal models provides indirect support for this hypothesis, and studies showing that CSD is common in humans with brain injury clearly demonstrate that the phenomenon can occur in the human brain. Considerable uncertainty about the role of CSD in migraine remains, however, and key questions about how this event is initiated, how it spreads, and how it might cause migraine symptoms remain unanswered. This Review summarizes current concepts of CSD and its potential roles in migraine, and addresses ongoing studies aimed at a clearer understanding of this fundamental brain phenomenon.

Migraine strikes as neuronal excitability reaches a tipping point

Self-propagating waves of cerebral neuronal firing, known as spreading depolarisations, are believed to be at the roots of migraine attacks. We propose that the start of spreading depolarisations corresponds to a critical transition that occurs when dynamic brain networks approach a tipping point. We show that this hypothesis is consistent with current pathogenetic insights and observed dynamics. Our view implies that migraine strikes when modulating factors further raise the neuronal excitability in genetically predisposed subjects to a level where even minor perturbations can trigger spreading depolarisations. A corollary is that recently discovered generic early warning indicators for critical transitions may be used to predict the onset of migraine attacks even before patients are clinically aware. This opens up new avenues for dissecting the mechanisms for the onset of migraine attacks and for identifying novel prophylactic treatment targets for the prevention of attacks.

Genes and primary headaches: discovering new potential therapeutic targets

Genetic studies have clearly shown that primary headaches (migraine, tension-type headache and cluster headache) are multifactorial disorders characterized by a complex interaction between different genes and environmental factors. Genetic association studies have highlighted a potential role in the etiopathogenesis of these disorders for several genes related to vascular, neuronal and neuroendocrine functions. A potential role as a therapeutic target is now emerging for some of these genes. The main purpose of this review is to describe new advances in our knowledge regarding the role of MTHFR, KCNK18, TRPV1, TRPV3 and HCRTR genes in primary headache disorders. Involvement of these genes in primary headaches, as well as their potential role in the therapy of these disorders, will be discussed.

Behavioral evidence for photophobia and stress-related ipsilateral head pain in transgenic Cacna1a mutant mice

Migraine is a highly prevalent, disabling and complex episodic brain disorder whose pathogenesis is poorly understood, due in part to the lack of valid animal models. Here we report behavioral evidence of hallmark migraine features, photophobia and unilateral head pain, in transgenic knock-in mice bearing human familial hemiplegic migraine, type 1 (FHM-1) gain-of-function missense mutations (R192Q or S218L) in the Cacna1a gene encoding the CaV2.1 calcium channel α1 subunit. Photophobia was demonstrated using a modified elevated plus maze in which the safe closed arms were brightly illuminated; mutant mice avoided the light despite showing no differences in the standard (anxiety) version of the test. Multiple behavioral measures suggestive of spontaneous head pain were found in 192Q mutants subjected to novelty and/or restraint stress. These behaviors were: (1) more frequent in mutant versus wildtype mice; (2) lateralized in mutant but not in wildtype mice; (3) more frequent in females versus males; and (4) dose-dependently normalized by systemic administration of 2 different acute analgesics, rizatriptan and morphine. Furthermore, some of these behaviors were found to be more frequent and severe in 218L compared to 192Q mutants, consistent with the clinical presentation in humans. We suggest that Cacna1a transgenic mice can experience migraine-related head pain and can thus serve as unique tools to study the pathogenesis of migraine and test novel antimigraine agents.

Pathophysiology of migraine

Migraine is a collection of perplexing neurological conditions in which the brain and its associated tissues have been implicated as major players during an attack. Once considered exclusively a disorder of blood vessels, compelling evidence has led to the realization that migraine represents a highly choreographed interaction between major inputs from both the peripheral and central nervous systems, with the trigeminovascular system and the cerebral cortex among the main players. Advances in in vivo and in vitro technologies have informed us about the significance to migraine of events such as cortical spreading depression and activation of the trigeminovascular system and its constituent neuropeptides, as well as about the importance of neuronal and glial ion channels and transporters that contribute to the putative cortical excitatory/inhibitory imbalance that renders migraineurs susceptible to an attack. This review focuses on emerging concepts that drive the science of migraine in both a mechanistic direction and a therapeutic direction.

Calcium channels and migraine

Missense mutations in CACNA1A, the gene that encodes the pore-forming α1 subunit of human voltage-gated Ca(V)2.1 (P/Q-type) calcium channels, cause a rare form of migraine with aura (familial hemiplegic migraine type 1: FHM1). Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache that may be preceded by transient neurological aura symptoms. This review, first, briefly summarizes current understanding of the pathophysiological mechanisms that are believed to underlie migraine headache, migraine aura and the onset of a migraine attack, and briefly describes the localization and function of neuronal Ca(V)2.1 channels in the brain regions that have been implicated in migraine pathogenesis. Then, the review describes and discusses i) the functional consequences of FHM1 mutations on the biophysical properties of recombinant human Ca(V)2.1 channels and native Ca(V)2.1 channels in neurons of knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene, and ii) the functional consequences of these mutations on neurophysiological processes in the cerebral cortex and trigeminovascular system thought to be involved in the pathophysiology of migraine, and the insights into migraine mechanisms obtained from the functional analysis of these processes in FHM1 knockin mice. This article is part of a Special Issue entitled: Calcium channels.

Critical role of calcitonin gene-related peptide receptors in cortical spreading depression

Cortical spreading depression (CSD) is a key pathogenetic step in migraine with aura. Dysfunctions of voltage-dependent and receptor-operated channels have been implicated in the generation of CSD and in the pathophysiology of migraine. Although a known correlation exists between migraine and release of the calcitonin gene-related peptide (CGRP), the possibility that CGRP is involved in CSD has not been examined in detail. We analyzed the pharmacological mechanisms underlying CSD and investigated the possibility that endogenous CGRP contributes to this phenomenon. CSD was analyzed in rat neocortical slices by imaging of the intrinsic optical signal. CSD was measured as the percentage of the maximal surface of a cortical slice covered by the propagation of intrinsic optical signal changes during an induction episode. Reproducible CSD episodes were induced through repetitive elevations of extracellular potassium concentration. AMPA glutamate receptor antagonism did not inhibit CSD, whereas NMDA receptor antagonism did inhibit CSD. Blockade of voltage-dependent sodium channels by TTX also reduced CSD. CSD was also decreased by the antiepileptic drug topiramate, but not by carbamazepine. Interestingly, endogenous CGRP was released in the cortical tissue in a calcium-dependent manner during CSD, and three different CGRP receptor antagonists had a dose-dependent inhibitory effect on CSD, suggesting a critical role of CGRP in this phenomenon. Our findings show that both glutamate NMDA receptors and voltage-dependent sodium channels play roles in CSD. They also demonstrate that CGRP antagonism reduces CSD, supporting the possible use of drugs targeting central CGRP receptors as antimigraine agents.

Genetics of migraine: an update

An important genetic component of migraine was systematically established by epidemiological studies in the 1990s. Over the past 15 years, significant progress has been made in unraveling the genetic basis and pathophysiological mechanisms of familial hemiplegic migraine, a rare and severe autosomal-dominant subtype of migraine with aura. Three different causative genes (CACNA1A, ATP1A2 and SCN1A), all of which are involved in cerebral ion translocation, have been identified. Functional studies and mouse models have shown that mutations in these genes, by different mechanisms, cause a disturbed cerebral glutamate homeostasis and, thus, increase susceptibility to cortical spreading depression, the likely correlate of migraine aura. More recently, genome-wide association studies have, for the first time, detected robust risk variants associated with the more common, genetically complex types of migraine, which has generated new perspectives for genetic research in migraine. This review summarizes the current knowledge about migraine genetics, with a focus on both familial hemiplegic migraine and recent results of genome-wide association studies.

Involvement of AMPA receptors in CSD-induced impairment of LTP in the hippocampus

OBJECTIVE

To investigate the alteration of hippocampal long-term plasticity and basal synaptic transmission induced by repetitive cortical spreading depressions (CSDs).

BACKGROUND

There is a relationship between migraine aura and amnesia attack. CSD, a state underlying migraine attacks, may be responsible for hippocampus-related symptoms. However, the precise role of CSD on hippocampal activity has not been investigated.

METHODS

Male Wistar rats were divided into CSD and control groups. Repetitive CSDs were induced in vivo by topical application of solid KCl. Forty-five minutes later, the ipsilateral hippocampus was removed, and hippocampal slices were prepared for a series of electrophysiological studies.

RESULTS

Repetitive CSDs led to a decrease in the magnitude of long-term potentiation in the hippocampus. CSD also reduced hippocampal synaptic efficacy, as shown by a reduction in post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses. In contrast, the post-synaptic N-methyl-d-aspartate receptor responses remained unchanged. In addition, there were no changes in paired-pulse profiles between the groups, indicating that CSD did not induce any presynaptic alterations.

CONCLUSION

These findings suggest that a reduction of post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses is the mechanism responsible for impaired hippocampal long-term potentiation induced by CSD.

Understanding migraine through the lens of maladaptive stress responses: a model disease of allostatic load

The brain and body respond to potential and actual stressful events by activating hormonal and neural mediators and modifying behaviors to adapt. Such responses help maintain physiological stability ("allostasis"). When behavioral or physiological stressors are frequent and/or severe, allostatic responses can become dysregulated and maladaptive ("allostatic load"). Allostatic load may alter brain networks both functionally and structurally. As a result, the brain's responses to continued/subsequent stressors are abnormal, and behavior and systemic physiology are altered in ways that can, in a vicious cycle, lead to further allostatic load. Migraine patients are continually exposed to such stressors, resulting in changes to central and peripheral physiology and function. Here we review how changes in brain states that occur as a result of repeated migraines may be explained by a maladaptive feedforward allostatic cascade model and how understanding migraine within the context of allostatic load model suggests alternative treatments for this often-debilitating disease.

Imaging mass spectrometry to visualize biomolecule distributions in mouse brain tissue following hemispheric cortical spreading depression

MALDI mass spectrometry can simultaneously measure hundreds of biomolecules directly from tissue. Using essentially the same technique but different sample preparation strategies, metabolites, lipids, peptides and proteins can be analyzed. Spatially correlated analysis, imaging MS, enables the distributions of these biomolecular ions to be simultaneously measured in tissues. A key advantage of imaging MS is that it can annotate tissues based on their MS profiles and thereby distinguish biomolecularly distinct regions even if they were unexpected or are not distinct using established histological and histochemical methods e.g. neuropeptide and metabolite changes following transient electrophysiological events such as cortical spreading depression (CSD), which are spreading events of massive neuronal and glial depolarisations that occur in one hemisphere of the brain and do not pass to the other hemisphere , enabling the contralateral hemisphere to act as an internal control. A proof-of-principle imaging MS study, including 2D and 3D datasets, revealed substantial metabolite and neuropeptide changes immediately following CSD events which were absent in the protein imaging datasets. The large high dimensionality 3D datasets make even rudimentary contralateral comparisons difficult to visualize. Instead non-negative matrix factorization (NNMF), a multivariate factorization tool that is adept at highlighting latent features, such as MS signatures associated with CSD events, was applied to the 3D datasets. NNMF confirmed that the protein dataset did not contain substantial contralateral differences, while these were present in the neuropeptide dataset.

Migraine: a disorder of brain excitatory-inhibitory balance?

Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache and interictal hypersensitivity to sensory stimuli. Migraine arises from a primary brain dysfunction that leads to episodic activation and sensitization of the trigeminovascular pain pathway and as a consequence to headache. Major open issues concern the molecular and cellular mechanisms of the primary brain dysfunction(s) and of migraine pain. We review here our current understanding of these mechanisms, focusing on recent advances regarding migraine genetics, headache mechanisms, and the primary brain dysfunction(s) underlying migraine onset and susceptibility to cortical spreading depression, the neurophysiological correlate of migraine aura. We also discuss insights obtained from the functional analysis of familial hemiplegic migraine mouse models.

Pharmacological targeting of spreading depression in migraine

Migraine, particularly with aura, is a genetically heterogeneous disorder of ion channels, pumps or transporters associated with increased cortical excitability. Spreading depression, as one reflection of hyperexcitability, is the electrophysiological event underlying aura symptoms and a trigger for headache. Endogenous (e.g., genes and hormones) and exogenous factors (e.g., drugs) modulating migraine susceptibility have also been shown to modulate spreading depression susceptibility concordantly, suggesting that spreading depression can be a relevant therapeutic target in migraine. In support of this, several migraine prophylactic drugs used in clinical practice have been shown to suppress spreading depression susceptibility as a probable mechanism of action, despite belonging to widely different pharmacological classes. Hence, susceptibility to spreading depression can be a useful preclinical model with good positive and negative predictive value for drug screening.

Cortical spreading depression impairs oxygen delivery and metabolism in mice

Cortical spreading depression (CSD) is associated with severe hypoperfusion in mice. Using minimally invasive multimodal optical imaging, we show that severe flow reductions during and after spreading depression are associated with a steep decline in cerebral metabolic rate of oxygen. Concurrent severe hemoglobin desaturation suggests that the oxygen metabolism becomes at least in part supply limited, and the decrease in cortical blood volume implicates vasoconstriction as the mechanism. In support of oxygen supply-demand mismatch, cortical nicotinamide adenine dinucleotide (NADH) fluorescence increases during spreading depression for at least 5 minutes, particularly away from parenchymal arterioles. However, modeling of tissue oxygen delivery shows that cerebral metabolic rate of oxygen drops more than predicted by a purely supply-limited model, raising the possibility of a concurrent reduction in oxygen demand during spreading depression. Importantly, a subsequent spreading depression triggered within 15 minutes evokes a monophasic flow increase superimposed on the oligemic baseline, which markedly differs from the response to the preceding spreading depression triggered in naive cortex. Altogether, these data suggest that CSD is associated with long-lasting oxygen supply-demand mismatch linked to severe vasoconstriction in mice.

Glial Hsp70 protects K+ homeostasis in the Drosophila brain during repetitive anoxic depolarization

Neural tissue is particularly vulnerable to metabolic stress and loss of ion homeostasis. Repetitive stress generally leads to more permanent dysfunction but the mechanisms underlying this progression are poorly understood. We investigated the effects of energetic compromise in Drosophila by targeting the Na(+)/K(+)-ATPase. Acute ouabain treatment of intact flies resulted in subsequent repetitive comas that led to death and were associated with transient loss of K(+) homeostasis in the brain. Heat shock pre-conditioned flies were resistant to ouabain treatment. To control the timing of repeated loss of ion homeostasis we subjected flies to repetitive anoxia while recording extracellular [K(+)] in the brain. We show that targeted expression of the chaperone protein Hsp70 in glial cells delays a permanent loss of ion homeostasis associated with repetitive anoxic stress and suggest that this is a useful model for investigating molecular mechanisms of neuroprotection.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.